For several decades, electronic circuitry has been shrinking at a famously dizzying pace. Too bad the batteries that typically power those circuits have not managed to get much smaller at all.
In today's wrist-worn GPS receivers, matchbox-size digital cameras, and pocketable personal computers, batteries are a significant portion of the volume. And yet they don't provide nearly enough energy, conking out seemingly at the worst possible moment.
The reason is simple: batteries are still little cans of chemicals. They function in essentially the same way they did two centuries ago, when the Italian physicist Alessandro Volta sandwiched zinc and silver disks to create the first chemical battery, which he used to make a frog's leg kick.
Now, with technologists busily ushering in a new age of miniaturization based on microelectromechanical systems (MEMS), batteries have arrived at a critical juncture. MEMS are finding applications in everything from the sensors in cars that trigger air bags to injectable drug delivery systems to environmental monitoring devices. Many of these systems ideally have to work for long periods, and it is not always easy to replace or recharge their batteries. So to let these miniature machines really hit their stride, we'll need smaller, longer-lasting power sources.
For several years our research groups at Cornell University and the University of Wisconsin-Madison have been working on a way around this power-source roadblock: harvesting the incredible amount of energy released naturally by tiny bits of radioactive material.
The microscale generators we are developing are not nuclear reactors in miniature, and they don't involve fission or fusion reactions. All energy comes from high-energy particles spontaneously emitted by radioactive elements. These devices, which we call nuclear microbatteries, use thin radioactive films that pack in energy at densities thousands of times greater than those of lithium-ion batteries [see table, ].
A speck of a radioisotope like nickel-63 or tritium, for example, contains enough energy to power a MEMS device for decades, and to do it safely. The particles these isotopes emit, unlike more energetic particles released by other radioactive materials, are blocked by the layer of dead skin that covers our bodies. They penetrate no more than 25 micrometers in most solids or liquids, so in a battery they could safely be contained by a simple plastic package [see sidebar, "Not All Radioisotopes Are Equal"].
Our current prototypes are still relatively big, but like the first transistors they will get smaller, going from macro- to microscale devices. And if the initial applications powering MEMS devices go well, along with the proper packaging and safety considerations, lucrative uses in handheld devices could be next. The small nuclear batteries may not be able to provide enough electric current for a cellphone or a PDA, but our experiments so far suggest that several of these nuclear units could be used to trickle charges into the conventional chemical rechargeable batteries used in handheld devices. Depending on the power consumption of these devices, this trickle charging could enable batteries to go for months between recharges, rather than days, or possibly even to avoid recharges altogether.